Integrated Circuit Structure with Source/Drain Spacers

ABSTRACT

The device includes a semiconductor substrate and a stack of channel layers on the semiconductor substrate. A top surface of a topmost channel layer extends along a first height relative to the substrate surface. A bottom surface of a bottommost channel layer extends along a second height relative to the substrate surface. The device further includes a gate structure that engages with the stack of channel layers and extending along a first direction. Additionally, the device includes a source/drain feature on first sidewall surfaces of the stack of channel layers and on the substrate, where the first sidewall surfaces extends in parallel to the first direction. Moreover, the source/drain feature has a first width along the first direction at the first height and a second width along the first direction at the second height, and wherein the first width is greater than the second width.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing, and for these advancements to berealized, similar developments in IC processing and manufacturing areneeded.

For example, nano-sheet-based devices have been introduced in an effortto improve gate control by increasing gate-channel coupling, reduceOFF-state current, and reduce short-channel effects (SCEs).Nano-sheet-based devices include a plurality of channel layers stackedtogether to form the transistor channels which are engaged by a gatestructure. The nano-sheet-based devices are compatible with conventionalcomplementary metal-oxide-semiconductor (CMOS) processes, allowing themto be aggressively scaled down while maintaining gate control andmitigating SCEs. However, due to the complex device structures, it maybe challenging to strike a balance between an optimal current densityand a low fringe capacitance. Therefore, although conventionalnano-sheet-based devices have been generally adequate for their intendedpurposes, they are not satisfactory in every respect.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIGS. 1A, 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, and 10A illustrate threedimensional views of an example workpiece of the present disclosure atvarious fabrication stages according various aspects of the presentdisclosure.

FIGS. 1B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, 9B, and 10B illustrate fragmentarycross-sectional views of an example workpiece of the present disclosurealong the line B-B′ in FIGS. 1A, 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, and10A, respectively, according various aspects of the present disclosure.

FIGS. 9B′ and 9B″ illustrate fragmentary cross-sectional views of anexample workpiece of the present disclosure along the line B-B′ in FIG.9A, according various aspects of the present disclosure.

FIGS. 3C, 4C, 5C, 6C, 7C, 8C, 9C, and 10C illustrate fragmentarycross-sectional views of an example workpiece of the present disclosurealong the line C-C′ in FIGS. 3A, 4A, 5A, 6A, 7A, 8A, 9A, and 10A,respectively, according various aspects of the present disclosure.

FIGS. 3D, 4D, 5D, 6D, 7D, 8D, 9D, and 10D illustrate fragmentarycross-sectional views of an example workpiece of the present disclosurealong the line D-D′ in FIGS. 3A, 4A, 5A, 6A, 7A, 8A, 9A, and 10A,respectively, according various aspects of the present disclosure.

FIGS. 7E, 8E, 9E, and 10E illustrate fragmentary cross-sectional viewsof an example workpiece of the present disclosure along the line E-E′ inFIGS. 7A, 8A, 9A, and 10A, respectively, according various aspects ofthe present disclosure.

FIGS. 7F, 8F, 9F, and 10F illustrate fragmentary cross-sectional viewsof an example workpiece of the present disclosure along the line F-F′ inFIGS. 7A, 8A, 9A, and 10A, respectively, according various aspects ofthe present disclosure.

FIGS. 7G, 8G, 9G, and 10G illustrate fragmentary cross-sectional viewsof an example workpiece of the present disclosure along the line G-G′ inFIGS. 7A, 8A, 9A, and 10A, respectively, according various aspects ofthe present disclosure.

FIG. 11 illustrate fragmentary cross-sectional views of an exampleworkpiece of the present disclosure along according various aspects ofthe present disclosure.

FIG. 12 is a flow chart of an example method of the present disclosureaccording various aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are within+/−10% of the number described, unless otherwise specified. For example,the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5nm.

The present disclosure is generally related to ICs and semiconductordevices and methods of forming the same. More particularly, the presentdisclosure is related to vertically-stacked horizontally-orientedmulti-channel transistors, such as nanowire transistors and nanosheettransistors. These types of transistors are sometimes referred to asgate-all-around (GAA) transistors, multi-bridge-channel (MBC)transistors, or some other names. In the present disclosure, they arebroadly referred to as nano-sheet-based transistors (or transistors, orsimply devices). A nano-sheet-based device includes a plurality ofchannel layers stacked one on top of another and engaged by a gatestructure. The channel layers of a nano-sheet-based device may includeany suitable shapes and/or configurations. For example, the channellayers may be in one of many different shapes, such as wire (ornanowire), sheet (or nanosheet), bar (or nano-bar), and/or othersuitable shapes. In other words, the term nano-sheet-based devicesbroadly encompasses devices having channel layers in nanowire,nano-bars, and any other suitable shapes. Further, the channel layers ofthe nano-sheet-based devices may engage with a single, contiguous gatestructure, or multiple gate structures. The channel layers connect apair of source/drain features, such that the charge carriers may flowfrom the source region to the drain region through the channel layersduring the operation (such as when the transistors are turned on).Additionally, inner spacers are formed between the source/drain featuresand the gate structures such that the source/drain features may beshielded from the operations targeting the gate structure. Thenano-sheet based devices presented herein may be a complementarymetal-oxide-semiconductor (CMOS) device, a p-typemetal-oxide-semiconductor (PMOS) device, or an n-typemetal-oxide-semiconductor (NMOS) device. One of ordinary skill mayrecognize other examples of semiconductor devices that may benefit fromaspects of the present disclosure. Moreover, although the disclosureuses nano-sheet-based devices as an example, one of ordinary skill mayrecognize other examples of semiconductor devices that may benefit fromaspects of the present disclosure. For example, other types ofmetal-oxide semiconductor field effect transistors (MOSFETs), such asplanar MOSFETs, FinFETs, other multi-gate FETs may benefit from aspectsof the present disclosure.

In a typical nano-sheet-based device, the source/drain features growfrom and cover the entirety of sidewall surfaces of the channel layers.This ensures the full capacity of the channel layers for conductivityare utilized in operation. Moreover, the growths of the source/drainfeatures often extend laterally beyond edges of the channel layers (e.g.along the lengthwise direction of the gate structures) such that thesource/drain features span a greater width than the channel layersthemselves. This larger lateral width does not improve the chargeconductivity of the transistor, but does provide a larger landingplatform for subsequently formed contact features which helps reduce thecontact resistance therebetween. However, the increased lateraldimension of the source/drain features, as compared to the channellayers, also contributes to an increased fringe capacitance of thedevice, which offsets the described benefits and sometimes evenadversely impacts the device performances. Accordingly, the presentdisclosure provides methods that allow formation of source/drainfeatures having a narrower bottom portion and a wider top portion, suchthat the contact resistance between the source/drain features and thecontact features is minimized while the fringe capacitance does notunnecessarily increase.

The various aspects of the present disclosure will now be described inmore detail with reference to the figures. FIGS. 1A-10A illustratethree-dimensional (3D) views of a workpiece 200 at different stages offabrication according to embodiments of the method of the presentdisclosure. FIGS. 1B-10B and 11 illustrate cross-sectional views of theworkpiece 200 (such as of an X-Z cross-section along the B-B′ line ofthe corresponding FIGS. 1A-10A). FIGS. 3C-10C, 3D-10D, 7E-10E, 7F-10F,and 7G-10G illustrate fragmentary cross-sectional views of an exampleworkpiece 200 of the present disclosure along the line C-C′, the lineD-D′, the line E-E′, the line F-F′, and the line G-G′ in FIGS. 3A-10A,respectively, according various aspects of the present disclosure. FIG.12 illustrates a flowchart of a method 100 for forming a semiconductordevice 200 from a workpiece 200 according to one or more aspects of thepresent disclosure. Method 100 is merely an example and is not intendedto limit the present disclosure to what is explicitly illustrated inmethod 100. Additional steps may be provided before, during and aftermethod 100, and some steps described can be replaced, eliminated, ormoved around for additional embodiments of the method. Not all steps aredescribed herein in detail for reasons of simplicity. Similarly, FIGS.1A-10A, 1B-10B, 3C-10C, 3D-10D, 7E-10E, 7F-10F, 7G-10G and 11 have beenabbreviated for simplicity and clarity, and may not include allfeatures.

Referring to FIGS. 1A, 1B, and to block 102 of method 100, a workpiece200 is received (or provided). The workpiece 200 includes a substrate202 and a stack of semiconductor layers 204 disposed on the substrate202. In some embodiments, the substrate 202 may be a silicon (Si)substrate. In some other embodiments, the substrate 202 may includeother semiconductors such as germanium (Ge), silicon germanium (SiGe),or a III-V semiconductor material. Example III-V semiconductor materialsmay include gallium arsenide (GaAs), indium phosphide (InP), galliumphosphide (GaP), gallium nitride (GaN), gallium arsenide phosphide(GaAsP), aluminum indium arsenide (AlInAs), aluminum gallium arsenide(AlGaAs), gallium indium phosphide (GaInP), and indium gallium arsenide(InGaAs). The substrate 202 may also include an insulating layer, suchas a silicon oxide layer, to have a silicon-on-insulator (SOI) structureor a germanium-on-insulator (GOI) structure. In some embodiments, thesubstrate 202 may include one or more well regions, such as n-type wellregions doped with an n-type dopant (i.e., phosphorus (P) or arsenic(As)) or p-type well regions doped with a p-type dopant (i.e., boron(B)), for forming different types of devices. The doping the n-typewells and the p-type wells may be formed using ion implantation orthermal diffusion.

The stack of semiconductor layers 204 may include a plurality of channellayers 208 interleaved (or interweaved) by a plurality of sacrificiallayers 206. The channel layers 208 and the sacrificial layers 206 mayhave different semiconductor compositions. In some implementations, thechannel layers 208 are formed of silicon (Si), such as crystalline Si,and sacrificial layers 206 are formed of silicon germanium (SiGe). Inthese implementations, the additional germanium content in thesacrificial layers 206 allow selective removal or recess of thesacrificial layers 206 without substantial damages to the channel layers208. In some embodiments, the sacrificial layers 206 and channel layers208 may be deposited using an epitaxial process. The stack ofsemiconductor layers 204 may be epitaxially deposited using ChemicalVapor Deposition (CVD) techniques (e.g., vapor-phase epitaxy (VPE)and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy (MBE),and/or other suitable processes. The sacrificial layers 206 and thechannel layers 208 are deposited alternatingly, one-after-another, toform the stack 204. For patterning purposes, the workpiece 200 may alsoinclude hard mask layers 209 over the stack 204. The hard mask layers209 may be a single layer or a multilayer. In some embodiments, the hardmask layers 209 are formed of silicon nitride. As described later, thehard mask layers 209 also protects the channel layers in subsequentetching operations. In some embodiments, additional pad oxide layer 207may be optionally formed between the hard mask layer 209 and the topmostchannel layers 208.

In some embodiments, the channel layers 208 each have a thickness 402 ofabout 3 nm to about 15 nm, such as about 5 nm to about 10 nm. If thethickness 402 is too small, the migration of the charge carriers throughthe channel layers 208 may become the bottleneck that restricts thedevice performance. If the thickness 402 is too large, the gate may noteffectively control all portions of the channel layers 208. Thesacrificial layers 206 each have a thickness 404 of about 3 nm to about15 nm, such as about 5 nm to about 10 nm. If the thickness 404 is toosmall, there may not be sufficient space to subsequently form allnecessary gate layers between adjacent channel layers 208. If thethickness 404 is too large, any additional benefit is offset by theextra processing and material costs. It is noted that three (3) layersof the sacrificial layers 206 and four (4) layers of the channel layers208 are alternately and vertically arranged as illustrated in FIGS. 1Aand 1B, which are for illustrative purposes only and not intended to belimiting. The number of layers depends on the desired number of channelsfor the semiconductor device. In some embodiments, the number of thechannel layers 208 is between 2 and 10. The distance between a topsurface of a topmost channel layer 208 and a bottom surface of abottommost channel layer 208 (or a top surface of the substrate 202) mayalso be referred to as a stack height 412. The stack height isdetermined by the number of channel layers 208, the thickness(es) of thechannel layers, the number of sacrificial layers 206, and thethickness(es) of the sacrificial layers. In some embodiments, the stackheight 412 may be about 35 nm to about 65 nm. If the stack height is toosmall, such as less than 35 nm, there may be insufficient number orthickness of channel layers 208 to be formed in the transistor, suchthat the conductive path for the operation current may be unnecessarilyrestricted. If the stack height is too large, such as greater than 65nm, the additional layers and/or the greater thickness may notsufficiently justify their fabrication cost and/or the physical spacethey occupy.

In some embodiments, the hard mask layers 209 (or collectively with thepad oxide layers 207, if present) have a thickness 414. As describedlater, the thickness 414 determines the height of a subsequently formedhigh-k hard mask layer, which forms part of the cut-metal-gatedielectric feature. In some embodiments, the thickness 414 may be about10 nm to about 40 nm, such as about 15 nm to about 30 nm. If thethickness 414 is too small, such as less than about 10 nm, thesubsequently formed cut-metal-gate dielectric feature may not besufficient height to cut through the height of the gate structure.Conversely, if the thickness 414 is too large, such as greater thanabout 40 nm, the additional height does not bring substantial benefityet occupies valuable device space.

Still referring to FIGS. 1A and 1B, the stack of semiconductor layers204 and the substrate 202 immediately therebeneath have been patternedto form fin-shaped structure 212 (interchangeable referred to as activeregions 212 or fin-active regions 212) using a patterning operation.Each of the fin-shaped structures 212 includes a base portion 212Bformed from a portion of the substrate 202 and a stack portion 212Sformed from the stack 204. The stack portion 212S is disposed over thebase portion 212B. The fin-shaped structures 212 extend lengthwise alongthe Y-direction and extend vertically along the Z-direction upwards fromthe substrate 202. The fin-shaped structures 212 may be patterned usingsuitable processes including double-patterning or multi-patterningprocesses. Generally, double-patterning or multi-patterning processescombine photolithography and self-aligned processes, allowing patternsto be created that have, for example, pitches smaller than what isotherwise obtainable using a single, direct photolithography process.For example, in one embodiment, a material layer is formed over asubstrate and patterned using a photolithography process. Spacers areformed alongside the patterned material layer using a self-alignedprocess. The material layer is then removed, and the remaining spacers,or mandrels, may then be used to pattern the fin-shaped structures 212by etching the stack 204 and the substrate 202. The etching process caninclude dry etching, wet etching, reactive ion etching (RIE), and/orother suitable processes. In some embodiments, additional hard masklayers (such as oxide hard mask layer) may have been formed on top ofthe hard mask layer 209 prior to the patterning of the fin-shapedstructures (such as prior to forming the mandrels). Those additionalhard mask layers are removed following the completion of the patterningprocess. In some embodiments, the fin-shaped structures 212 areconfigured to have a width 408 along the X-direction. In someembodiments, the width 408 may be about 20 nm to about 40 nm.

The workpiece 200 further includes an isolation feature 214 formedbetween the adjacent fin-shaped structures 212. The isolation feature214 may be formed by first depositing a precursor layer over theworkpiece 200 and filling spaces (or trenches) between the fin-shapedstructures 212 and subsequently recessed to expose at least the topportions of the fin-shaped structures 212. The dielectric material mayinclude silicon oxide, silicon nitride, silicon oxynitride,fluorine-doped silicate glass (FSG), a low-k dielectric, combinationsthereof, and/or other suitable materials. In various examples, thedielectric material may be deposited by a CVD process, a subatmosphericCVD (SACVD) process, a flowable CVD (FCVD) process, an ALD process,spin-on coating, and/or other suitable process. In some embodiments, aliner may be formed to wrap around the fin-shaped structures 212 priorto the formation of the isolation feature 214. Accordingly, as shown inFIGS. 1A and 1B, the stack portions 212S of the fin-shaped structures212 rise above the isolation feature 214 while the base portions 212Bare surrounded by the isolation feature 214. In some embodiments, thestack portions 212S have a height of about 45 nm to about 60 nm. If theheight is too small, such as less than about 45 nm, the current passagethrough the channel layers may be limited; while if the height is toolarge, such as greater than about 60 nm, the additional chip footprintmay not justify any performance improvements.

Referring to FIGS. 2A and 2B, method 100 includes a block 104 where acladding layer 216 is formed over the fin-shaped structures 212. In someembodiments, the cladding layer 216 may have a composition similar to,but different from, that of the sacrificial layers 206. In one example,the cladding layer 216 may be formed of silicon germanium (SiGe), andthe sacrificial layers 206 is also formed of SiGe. This commoncomposition allows efficient selective removal of the sacrificial layers206 and the cladding layer 216 without adversely affect the channellayers 208 (such as formed of Si) in a subsequent process (such as agate replacement process described below). However, the cladding layer216 is also configured to achieve an etching selectivity from thesacrificial layers 206 under another etching condition (such as thedielectric layer replacement process described below). For example, insome embodiments, the cladding layer 216 is formed of SiGe in anamorphous state, while the sacrificial layers 206 are formed ofcrystalline SiGe. For another example, the cladding layer 216 may have adifferent Ge atomic percentage than that of the sacrificial layers 206.For instance, the cladding layer 216 may include a Ge atomic percentageof about 15% to about 25%; while the sacrificial layer 206 may include aGe atomic percentage of about 20% to about 30%. In furtherance of thisinstance, the cladding layer 216 may include a Ge atomic percentage ofless than about 22.5%; while the sacrificial layer 206 may include a Geatomic percentage of greater than about 22.5%. In some embodiments, thecladding layer 216 may be conformally and epitaxially grown using VPE orMBE. In some alternative embodiments, the cladding layer 216 may bedeposited using CVD, ALD, other suitable deposition method, orcombinations thereof. As shown in FIGS. 2A and 2B, the cladding layer216 is selectively disposed on sidewalls of the fin-shaped structures212. The cladding layer 216 has a thickness 406. In some embodiments,the thickness 406 is about 2 nm to about 20 nm, for example, about 5 nmto about 15 nm. As described later, the thickness 406 determines thewidth of a subsequently formed trench in which a dielectric material isdeposited. If the thickness 406 is too small, it may be challenging tofully fill the subsequently formed trench. If the thickness is toolarge, the additional benefit may not justify the chip footprint itrequires.

Referring to FIGS. 3A and 3B, method 100 includes a block 106 where adielectric feature 218 (or dielectric barrier 218) is formed betweenadjacent fin-shaped structures 212. In some embodiments, the dielectricfeatures 218 separate adjacent fin structures 212 and in some instancesdefine lengths of subsequently formed gate structures. Dielectricfeatures 218 are sometimes implemented in cut-metal-gate (CMG)processes. The dielectric feature 218 may include multiple layers. Forexample, in the depicted embodiments, a filler layer 220 is depositedover the workpiece 200. In some embodiments, a composition of the fillerlayer 220 may be similar to a composition of the isolation feature 214.In some embodiments, the filler layer 220 may be deposited using a CVDprocess, an SACVD process, an FCVD process, an ALD process, a PVDprocess, spin-on coating, and/or other suitable process. The workpiece200 is then planarized. In some embodiments, the workpiece 200 isplanarized using a CMP process until top surfaces of the hard mask layer209 are exposed. In some embodiments, another layer 221 is formedconformally prior to the deposition of the filler layer 220 such that itinterposes between the filler layer 220 and the cladding layer 216, aswell as between the filler layer 220 and the isolation feature 214.Moreover, in some embodiments, the filler layer 220 is recessed, suchthat a top surface of the filler layer extends below a top surface ofthe hard mask layer 209. In some embodiments, the top surface of therecessed filler layer 220 is below the top surface of the topmostchannel layer 208 by about 4 nm to about 45 nm. Subsequently, adielectric layer 222 is formed over the recessed filler layer 220, onthe top surface of the hard mask layer 209, and on the top surface ofthe cladding layer 216. The dielectric layer 222 may be formed of high-kdielectric materials, and may be interchangeably referred to as thehigh-k dielectric layer 222 or high-k hard mask layer 222. As usedherein, high-k dielectric materials include dielectric materials havinga high dielectric constant, for example, greater than that of thermalsilicon oxide (˜3.9). The dielectric layer 222 may include hafniumoxide. Alternatively, the dielectric layer 222 may include other high-kdielectrics, such as titanium oxide (TiO₂), hafnium zirconium oxide(HfZrO), tantalum oxide (Ta₂O₅), hafnium silicon oxide (HfSiO₄),zirconium oxide (ZrO₂), zirconium silicon oxide (ZrSiO₂), lanthanumoxide (La₂O₃), aluminum oxide (Al₂O₃), zirconium oxide (ZrO), yttriumoxide (Y₂O₃), SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, hafnium lanthanum oxide(HfLaO), lanthanum silicon oxide (LaSiO), aluminum silicon oxide(AlSiO), hafnium tantalum oxide (HMO), hafnium titanium oxide (HfTiO),(Ba,Sr)TiO₃ (BST), silicon nitride (SiN), silicon oxynitride (SiON),combinations thereof, or other suitable material. In some embodiments, aCMP process is conducted to remove excess materials and to expose a topsurface of the hard mask layer 209. At this processing stage, therecessed filler layer 220, the layer 221, and the dielectric layer 222collectively form the dielectric feature 218 that isolates fin-shapedstructures 212 from one another. As illustrated in FIG. 3B, sidewallsurfaces of adjacent dielectric features 218 are spaced apart by adistance 410. The distance 410 equals to the sum of the width 408 andtwice the thickness 406 (or interchangeable referred to as the width406). As described in more detail below, the distance 410 determines thewidth of top portions of subsequently formed source/drain features, andtherefore is interchangeably referred to as the width 410.

Still referring to FIGS. 3A and 3B, method 100 includes a block 108where gate stacks 230 are formed on the fin-shaped structures 212. Insome embodiments, the gate stacks 230 extend orthogonally to thelengthwise direction of the fin-shaped structures 212. For example, inthe depicted embodiments, the gate stacks 230 extends along theX-direction. In some embodiments, a gate replacement process (orgate-last process) is later adopted where the gate stacks 230 serve asplaceholders for subsequently formed functional gate structures (ormetal gate). Accordingly, the gate stacks 230 may alternatively bereferred to as the dummy gate stacks 230. Other processes andconfiguration are possible. The gate stacks 230 are formed on (and insome embodiments directly contacting) top surfaces of the dielectriclayer 222, top surfaces of the hard mask layer 209, and directlycontacting top surfaces of the cladding layers 216. Accordingly, asillustrated in FIG. 3D, portions of the cladding layer 216 are buriedunder the gate stacks 230. Meanwhile, as illustrated in FIG. 3C,portions of the cladding layers 216 are exposed in regions betweenadjacent gate stacks 230. Each of the gate stacks 230 includes a gateelectrode (or a dummy gate electrode). In some embodiments, the gatestacks 230 may further include other layers such as gate dielectriclayers, interfacial layers, other suitable layers or combinationsthereof. Layers for the gate stacks 230 may be formed by any suitablemethods, such as CVD. In some embodiments, a gate top hard mask (notshown) is deposited over the material layer for the gate electrode whichassists the patterning of the gate electrode. The gate top hard mask maybe a multi-layer and include a silicon nitride mask layer and a siliconoxide mask layer over the silicon nitride mask layer. The material layerfor the gate electrodes is then patterned using photolithographyprocesses to form the gate electrodes. In some embodiments, the gateelectrodes may include polycrystalline silicon (polysilicon). In someembodiments, an oxide layer is formed interposing between the topsurfaces of the topmost channel layer 208 and the gate stacks 230.

Referring to FIGS. 4A-4D, method 100 includes a block 110 of FIG. 12where the cladding layer 216 between the dielectric feature 218 and thefin-shaped structures 212 are removed in a selective etching operation.In some embodiments, the selective etching operation is configured suchthat cladding layer 216 not covered by the gate stacks 230 is removed inits entirety without substantially etching the layer 221, the channellayers 208 or the sacrificial layers 206. Accordingly, trenches 316 areformed from the partial removal of the cladding layers 216. The trenches316 have dimensions substantially similar to the dimensions of thecladding layer 216. For example, the trenches 316 may have a width 406of about 5 nm to about 15 nm. The sidewall surfaces of the layer 221,sidewall surfaces of the channel layers 208, as well as the sidewallsurfaces of the sacrificial layers 206 are exposed in trenches 316.Meanwhile, portions of the cladding layer 216 covered under the gatestacks 230 remain intact, as illustrated in FIG. 4D.

Referring to FIGS. 5A-5D and to block 112 of FIG. 12, a dielectricmaterial is deposited into the trenches 316 to substantially fill thetrenches 316, thereby forming the dielectric features 1216 (alsoreferred to as the source/drain spacers). Accordingly, the dielectricfeatures 1216 have the width 406. Moreover, because the dielectricfeatures 1216 are formed on both ends of the fin-shaped structures 212,sidewall surfaces of adjacent dielectric features 1216 are spaced apartby the width 408 (alternatively and interchangeably referred to as thedistance 408). As described below, in some embodiments, the distance 408defines the dimension of lower portions of subsequently formedsource/drain features. The dielectric material may be selected based onat least the etching resistance against subsequently used etchingconditions. For example, in some embodiments, the dielectric materialmay serve the function of a spacer between the gate structure and theepitaxial source/drain features. For example, the dielectric materialmay be configured to resist etching conditions employed in source/draintrenches formation, channel releases (or sheet formations), and contacttrench etching operations. Accordingly, the dielectric material protectsthe subsequently formed source/drain features and preventsgate-to-source/drain shorting or leakage. In some embodiments, thedielectric material includes a low-k dielectric material. For example,the dielectric material may include silicon carbonitride (SiCN), siliconcarboxynitride (SiCON), silicon carbide (SiC), other suitable dielectricmaterials, or combinations thereof. The deposition of the dielectricmaterial may implement any suitable deposition techniques, such asPhysical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), AtomicLayer Deposition (ALD), other suitable deposition techniques, orcombinations thereof. In some embodiments, the dielectric material isinitially formed over top surfaces of the dielectric layer 222, over thetop surfaces of the hard mask layers 209, as well as on sidewallsurfaces of the gate stacks 230. Subsequently, an etching-back operationis conducted to remove the excess dielectric materials and to expose thetop surfaces of the dielectric layer 222, and to expose sidewallsurfaces of the gate stacks 230. Accordingly, as illustrated in FIG. 5B,remaining portions of the dielectric material becomes the dielectricfeatures 1216, where the top surfaces of the dielectric feature 1216 areexposed. The dielectric features 1216 may each have a height 416measured from a top surface of the substrate 202 (and a top surface ofthe isolation feature 214) to a top surface of the dielectric features1216. In some embodiments, the CMP operation results in a concaved topsurface of the dielectric features 1216 and the hard mask layer 209. Insuch embodiments, the height 416 refers to heights of the dielectricfeature 1216 averaged across its width dimension along the X-direction.In the depicted embodiments, the top surfaces of the dielectric features1216 extend below a top surface of the dielectric features 218 but abovea top surface of the filler layer 220. In some embodiments, an etchingoperation is conducted at this processing stage to recess the dielectricfeatures 1216. However, due to the restrictions of adjacent features, itmay be challenging to regulate and fine tune the sizes of the dielectricfeatures 1216. In the depicted embodiments, the height of the dielectricfeatures 1216 are adjusted at a later processing stage, as described inmore detail below.

During the etching-back operation, the hard mask layer 209 may bepartially removed. For example, the top surfaces of the hard mask layer209 may become concaved following the etching-back operation. Thepresence of the hard mask layers 209 protects the channel layers 208therebeneath to not be compromised. Following the completion of theetching-back operation, referring to FIGS. 6A-6D, the exposed portionsof the hard mask layer 209 are etched away selectively such thattrenches 219 are formed. For example, the hard mask layer 209 (and thepad oxide layer 207, if present) is removed using a wet etching processwithout substantially affecting the channel layers 208 thereunderneath.In some embodiments, the etching operation may implement a dry etching,a wet etching, or combinations thereof. In some embodiments, the etchingoperation is configured to form a substantially right angle (about 90°)between sidewalls of the gate stack 230 and the top surface of thechannel layer 208. This reduces formation of undesirable dielectricresidues following a subsequent spacer deposition. Meanwhile, the hardmask layers 209 covered under the gate stacks 230 are substantiallypreserved. The trenches 219 are defined by the sidewall surfaces of thedielectric features 1216 along the X-direction, and by the planes alongsidewall surfaces of the gate stacks 230 along the Y-direction. At thisfabrication stage, the top surfaces of the channel layers 208 areexposed in the trenches 219. Moreover, gate spacers 234 are formed alongsidewalls of the gate stacks 230. The gate spacers 234 may include oneor more gate spacer layers. The gate spacer 234 may include a dielectricmaterial, such as a dielectric material that allows selective removal ofthe gate stacks 230 without affecting the gate spacer 234. Suitabledielectric materials may include silicon nitride, siliconoxycarbonitride, silicon carbonitride, silicon oxide, siliconoxycarbide, silicon carbide, silicon oxynitride, and/or combinationsthereof. The gate spacer 234 may be conformally deposited over theworkpiece 200 using CVD, subatmospheric CVD (SACVD), ALD, other suitablemethods, or combinations thereof. For example, the gate spacers 234 areformed on top and sidewall surfaces of the gate stacks 230. In thedepicted embodiments, the gate spacers 234 extend into the trenches 219and cover sidewall surfaces of the remaining portions of the hard masklayer 209, and are further formed on and directly contact the topsurfaces of the topmost channel layers 208. Furthermore, the gatespacers 234 are formed on and directly contact the top surfaces of thedielectric features 1216, and on and directly contact the top surfacesof the dielectric layers 222. The gate stacks 230 and the gate spacers234 collectively form gate structures 232.

Referring to FIGS. 7A-7G and to block 114 of FIG. 12, the regions of thefin-shaped structures 212 underlying the gate structures 232 may bereferred to as channel regions 212C. Each of the channel regions 212C ina fin-shaped structure 212 is horizontally sandwiched between twosource/drain regions 212SD where source/drain features are subsequentlyformed. At this processing stage, the source/drain regions 212SD arerecessed to form source/drain trenches 236. With the gate structures 232serving as an etch mask, the workpiece 200 is anisotropically etched toform the source/drain trenches 236 in the source/drain regions 212SD. Insome embodiments as illustrated in FIG. 7B, operations at block 114 maysubstantially remove the stack portions 212S of fin-shaped structures212 in the source/drain regions 212SD, and the source/drain trenches 236may extend into the base portions 212B, which is formed from thesubstrate 202 (compare FIG. 6B). The anisotropic etch at block 128 mayinclude a dry etch process or a suitable etch process. For example, thedry etch process may implement an oxygen-containing gas, hydrogen, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof.The anisotropic etch at block 114 may implement a mask element. In someembodiments, the mask element covers the dielectric features 1216 suchthat they are preserved during the anisotropic etch. Accordingly, theheight 416 and width 406 of the dielectric features 1216 remainsunchanged. Accordingly, the source/drain trenches 236 are defined bysidewall surfaces of the dielectric features 1216 across theX-direction, and defined by planes along sidewall surfaces of the gatespacers 234 across the Y-direction. For example, the source/draintrenches 236 have a width 408 across the X-direction. At this processingstage, the source/drain trenches 236 may have a substantially uniformwidth (width 408) throughout their respective height along theZ-direction. Moreover, the anisotropic etch produces new sidewallsurfaces for the fin-shaped structures 212. Accordingly, new sidewallsurfaces of the channel layers 208 and the sacrificial layers 206 arenow exposed in the source/drain trenches 236 (see FIG. 7F).

Referring to FIGS. 8A-8D, the method 100 includes a block 116 whereinner spacer features 242 are formed. In some embodiments, thesacrificial layers 206 exposed in the source/drain trenches 236 areselectively and partially recessed to form inner spacer recesses,without substantially affecting the exposed channel layers 208. In anembodiment where the channel layers 208 consist essentially ofcrystalline silicon (Si) and sacrificial layers 206 consist essentiallyof silicon germanium (SiGe), the selective and partial recess of thesacrificial layers 206 may include a SiGe oxidation process followed bya SiGe-oxide removal. In such embodiments, the SiGe oxidation processmay include use of ozone, and the extent at which the sacrificial layers206 are recessed are determined by a time duration of the oxidationprocess. In some other embodiments, the selective recess may be aselective isotropic etching process (e.g., a selective dry etchingprocess or a selective wet etching process), and the extent at which thesacrificial layers 206 are recessed is controlled by duration of theetching process. The selective dry etching process may include use ofone or more fluorine-based etchants, such as fluorine gas orhydrofluorocarbons. The selective wet etching process may include ahydro fluoride (HF) or NH4OH etchant. In some embodiments, the selectiverecessing of the sacrificial layers 206 is configured such that theportion of the sacrificial layers 206 directly and vertically underneaththe gate spacers 234 are removed while the portion of the sacrificiallayers 206 under the gate stacks 230 are preserved. Subsequently, asillustrated in FIGS. 8C and 8F, an inner spacer material layer is thendeposited using CVD or ALD over the workpiece 200, including over andinto the inner spacer recesses. The inner spacer material may includesilicon nitride, silicon oxycarbonitride, silicon carbonitride, siliconoxide, silicon oxycarbide, silicon carbide, silico oxynitride, othersuitable materials, or combinations thereof. After the deposition of theinner spacer material layer, the inner spacer material layer is etchedback to form inner spacer features 242. Accordingly, in the depictedembodiments, the inner spacer features 242 are formed between endportions of the vertically adjacent channel layers 208 and betweenhorizontally adjacent dielectric features 1216 along the X-direction.Moreover, the inner spacer features 242 are formed vertically beneaththe gate spacers 234. The inner spacer features 242 isolates theremaining portions of the sacrificial layers 206 from areas wheresource/drain features are formed, such that subsequent etchingoperations on the sacrificial layers 206 do not affect the integrity ofthe source/drain features.

Referring to FIGS. 9A-9G, the method 100 includes a block 118 where thedielectric features 1216 are recessed along the Z-direction. Forexample, the height of the dielectric features 1216 are reduced from theheight 416 (see FIG. 8B) to the height 418 (or interchangeably referredto as the distance 418). In some embodiments, the recessed dielectricfeatures 1216 have a top surface that extends below a top surface of thedielectric features 218. For example, the recessed dielectric features1216 may have a top surface that extends below a top surface of thefiller layers 220. In some embodiments, the width 406 of the dielectricfeatures 1216 are substantially preserved. The recessing of thedielectric features 1216 may implement any suitable etching methods. Insome embodiments, dry etching methods are implemented. Moreover,parameters of the etching operation are adjusted to tune the height ofthe dielectric features 1216 according to the desired height 418 asdescribed below. For example, the time duration of the etching operationis adjusted in order to reach the desired height 418 and the desiredratio of the height 418 relative to the height 416.

In some embodiments, the height 418 may be about 5 nm to about 40 nm.For example, a ratio of the height 418 to the height 416 may be about0.05:1 to about 0.7:1. In some embodiments, the height 418 may be about10 nm to about 30 nm. For example, a ratio of the height 418 to theheight 416 may be about 0.1:1 to about 0.5:1. As described below, therecessed dielectric features assist the adjustment of the profiles ofthe subsequently formed source/drain features and contribute to reduceddevice fringe capacitances. If the height 418 is too small, such as lessthan about 5 nm, or if the ratio is too small, such as less than about0.05:1, the beneficial effect in capacitance reduction may be limited;if the height 418 is too large, such as greater than about 40 nm, or ifthe ratio is too larger, such as greater than about 0.7:1, the landingplatform for the subsequently formed contact features on thesource/drain features may be reduced and lead to increases in thecontact resistances. Meanwhile, because the top portions of thedielectric features 1216 are removed, sidewall surfaces of thedielectric features 218 are exposed in the source/drain trenches 236. Asdescribed above, sidewall surfaces of adjacent dielectric features 218are spaced away by the distance 410. The distance 410 determines thewidth of the top portions of source/drain features subsequently formedin the source/drain trenches 236 and is therefore interchangeablyreferred to as the width 410. Accordingly, the source/drain trenches 236each have a lower portion 236A having a width 408 across the X-directionand a top portion 236B having a width 410 across the X-direction. Insome embodiments, a ratio of the width 410 to the width 408 is about1.5:1 to about 2:1. If the ratio is too small, such as less than about1.5:1, any reduction in capacitances from forming the dielectricfeatures 1216 may be limited. Conversely, if the ratio is too large,such as greater than about 2:1, the additional benefit achieved may notsufficiently offset the extra processing costs. A distance between thetop surface of the recessed dielectric feature 1216 to the top surfaceof the dielectric layer 222 is referred to as the distance 420. Thedistance 420 is slightly less than the distance 416 (for example about1% to about 20% less).

Alternatively, referring to FIG. 9B′, in some embodiments, thedielectric features 1216 are not only vertically recessed, but alsolaterally recessed, such that the width of the remaining portions of thedielectric features 1216 is reduced from the width 406 (see FIG. 8B) tothe width 426. In such embodiments, the width of the lower portions 236Aof the source/drain trenches 236 is increased to the width 428, whilethe width of the higher portions 236B of the source/drain trenches 236stays as width 410. In some embodiments, the lateral recessing operationadjusts the lateral widths of the subsequently formed source/drainfeatures to achieve a designed size along the X-direction. For example,this allows fine-tuning of profiles of the source/drain featuressubsequently formed. As described below, in some embodiments, having agreater width 428 assists ensuring that the entire sidewall surfaces ofthe channel layers 208 are covered by the source/drain features, suchthat the full capacity of the channel layers 208 are utilized. In someembodiments, a ratio of the width 410 to the width 428 is about 1.7:1 toabout 4:1.

Accordingly, the present disclosure provides source/drain trenches 236having a stepped sidewall profile on the X-Z cross-section (e.g.perpendicular to the direction along which the fin-shaped structures 212extend), such that the source/drain trenches 236 have a wider opening atthe top than at the bottom. This configuration allows for minimizingcontact resistance between subsequently formed source/drain features andthe overlaying contact features, as well as for minimizing thecapacitances at the bottom portions of the source/drain features. Thepresent disclosure contemplates other methods for forming similarstepped profiles for the source/drain trenches 236 (and for thesubsequently formed source/drain features). For example, FIG. 9B″illustrates another embodiment, where the top portions of the dielectricfeatures 1216 are not removed, but rather laterally recessed to areduced width (such as width 429). Meanwhile, the lower portions of thedielectric features 1216 are not recessed, such that they retain thewidth 405. In some embodiments, a ratio of the width 429 to the width408 is about 1.2:1 to about 1.7:1. In still other embodiments, both topand bottom portions of the dielectric features 1216 may be laterallyrecessed, although to different extents, so as to form the steppedprofile with similar dimension ratios. The disclosures below proceedfrom the embodiment of FIG. 9B, although similar operations may proceedfrom other similar embodiments.

Referring to FIGS. 10A-10G, the method 100 includes a block 120 wheresource/drain features 245 are formed in the source/drain trenches 236and substantially fill majority of the source/drain trenches 236. Insome embodiments, the source/drain features 245 are selectively andepitaxially formed on the exposed sidewall surfaces of the channellayers 208 and on the exposed top surfaces of the substrate 202 in thesource/drain trenches 236. Moreover, the growth (or “overgrowth”) of thesource/drain features 245 from individual channel layers 208 and fromthe substrate 202 eventually merge over remaining portions of thesidewall surfaces of the source/drain trenches 236, such as oversidewall surfaces of the dielectric features 1216 and over sidewallsurfaces of the dielectric features 218. As described above, thesource/drain trenches 236 includes lower portions 236A having a smallerwidth (width 408) along the X-direction and top portions 236B having agreater width (width 410) along the X-direction. Accordingly, thesource/drain features 245 each include a lower portion 245A extendingbetween the two recessed dielectric features 1216, and a top portion245B over the lower portion 245A, as well as over top surfaces of therecessed dielectric features 1216. Moreover, the top portion 245B extendbetween and directly contacts sidewall surfaces of the dielectricfeatures 218. In the depicted embodiments, the lower portions 245A havethe height 418 and the width 428 (see FIG. 10B). In the depictedembodiments, the source/drain trench portions 236A and 236B each havesubstantially straight sidewalls. Accordingly, the widths of the lowerportions 245A of the source/drain features 245 and the widths of the topportions 245B of the source/drain features 245 are each substantiallyuniform. The width of the lower portions 245A along the X-direction maybe substantially similar to (or matching) the length of the channellayers 208 along the X-direction. Accordingly, the entire conductivecapacitance of the channel layers 208 is fully utilized without formingan excessively large lower portion 245A that tends to result in highercapacitances. Meanwhile, the width 410 of the top portions 245B isgreater than that of the lower portions 245A, such that the top surfaceof the top portions 245B have greater surface areas for better contactwith the subsequently formed contact features. In the depictedembodiments, the difference between the width 410 and the width 408 istwice the thickness (or width 406) of the dielectric features 1216.

Moreover, in the depicted embodiments, the top portions 245B has aheight 422. In other words, a top portion of the source/drain features245 (or the top portions 245B) extends above the top surfaces of thedielectric features 1216 by the distance 422. In some embodiments, thedistance 422 may be about 5 nm to about 40 nm. The distance 422 (or theheight 422) is less than the distance 420. Accordingly, the source/drainfeatures 245 have a height 424 that equals the sum of the distance 418and the distance 422. In some embodiments, a ratio of the distance 418to the distance 422 is about 0.1:1 to about 10:1, such as about 0.5:1 toabout 2:1 In other words, a ratio of the height 424 to the height 418 isabout 1.1:0.1 to about 11:10, such as about 1.5:0.5 to about 3:2. In theratio is too small, the beneficial effect in capacitance reduction maybe limited; if the ratio is too larger, the landing platform for thesubsequently formed contact features on the source/drain features may bereduced and lead to resistance increases. In some embodiments, the topportions 245B entirely cover and directly contact the top surface of thedielectric features 1216. In some embodiments, the growth of thesource/drain features 245 is configured to leave air gaps 270 betweenthe top surfaces of the dielectric features 1216 and the top portions245B of the source/drain features 245. In some embodiments, the air gaps270 help reducing the capacitances of the device.

The source/drain features 245 may be formed using an epitaxial process,such as vapor-phase epitaxy (VPE), ultra-high vacuum CVD (UHV-CVD),molecular beam epitaxy (MBE), and/or other suitable processes. Thesource/drain features 245 may be either n-type or p-type. When thesource/drain features 245 are n-type, they may include silicon (Si) andmay be doped with an n-type dopant, such as phosphorus (P) or arsenic(As). When the source/drain features 245 are p-type, they may includesilicon germanium (SiGe) or germanium (Ge) and may be doped with ap-type dopant, such as boron (B) or gallium (Ga). In some embodiments,the source/drain features 245 may each have multiple layers.

Referring to FIG. 11, the method 100 includes a block 122 where acontact etch stop layer (CESL) 243 and an interlayer dielectric (ILD)layer 244 are formed over the workpiece 200. In an example process, theCESL 243 is first conformally deposited over the workpiece 200 and thenthe ILD layer 244 is blanketly deposited over the CESL 243. The CESL 243may include silicon nitride, silicon oxide, silicon oxynitride, and/orother materials known in the art. The CESL 243 may be deposited usingALD, plasma-enhanced chemical vapor deposition (PECVD) process and/orother suitable deposition or oxidation processes. In some embodiments,the ILD layer 244 includes materials such as tetraethylorthosilicate(TEOS) oxide, un-doped silicate glass, or doped silicon oxide such asborophosphosilicate glass (BPSG), fused silica glass (FSG),phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/orother suitable dielectric materials. The ILD layer 244 may be depositedby spin-on coating, an FCVD process, or other suitable depositiontechnique. In some embodiments, after formation of the ILD layer 244,the workpiece 200 may be annealed to improve integrity of the ILD layer244. To remove excess materials and to expose top surfaces of the gateelectrode, a planarization process (such a chemical mechanical polishing(CMP) process) may be performed to the workpiece 200.

In some embodiments, the method 100 proceeds to a gate replacementprocess where the gate stacks 230 are replaced with functional gatestructures (see block 124 of FIG. 12). Gate replacement processes havebeen described in, for example, U.S. patent application Ser. No.16/657,606 filed on Oct. 18, 2019 to Jhon Jhy Liaw, the entirety ofwhich is incorporated herein by reference. For example, the method atblock 122 includes removing the gate stacks 230 to form a gate trench.Remaining portions of the sacrificial layers 206 between the channellayers 208 in the channel regions 212C are selectively removed from theexposed sidewalls in the gate trenches. Moreover, the remaining portionsof the cladding layer 216 under the gate stacks 230 are also removed.This process releases the channel layers 208 to form channel members.The channel members are vertically stacked along the Z direction. Theselective removal of the sacrificial layers 206 may be implemented byselective dry etch, selective wet etch, or other selective etchprocesses. In some embodiments, the selective wet etching includes anAPM etch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture). Insome alternative embodiments, the selective removal includes silicongermanium oxidation followed by a silicon germanium oxide removal. Forexample, the oxidation may be provided by ozone clean and then silicongermanium oxide removed by an etchant such as NH₄OH. An interfaciallayer and a gate dielectric layer are deposited in the gate trenches towrap around each of the channel members. In some embodiments, theinterfacial layer includes silicon oxide and may be formed as result ofa pre-clean process. An example pre-clean process may include use of RCASC-1 (ammonia, hydrogen peroxide and water) and/or RCA SC-2(hydrochloric acid, hydrogen peroxide and water). The pre-clean processoxidizes the exposed surfaces of the channel members to form interfaciallayer. The gate dielectric layer is then deposited over the interfaciallayer using ALD, CVD, and/or other suitable methods. The gate dielectriclayer may be formed of high-K dielectric materials. A gate electrodelayer is deposited in the gate trenches. The gate electrode layer may bea multi-layer structure that includes at least one work function layerand a metal fill layer. By way of example, the at least one workfunction layer may include titanium nitride (TiN), titanium aluminum(TiAl), titanium aluminum nitride (TiAlN), tantalum nitride (TaN),tantalum aluminum (TaAl), tantalum aluminum nitride (TaAlN), tantalumaluminum carbide (TaAlC), tantalum carbonitride (TaCN), or tantalumcarbide (TaC). The metal fill layer may include aluminum (Al), tungsten(W), nickel (Ni), titanium (Ti), ruthenium (Ru), cobalt (Co), platinum(Pt), tantalum silicon nitride (TaSiN), copper (Cu), other refractorymetals, or other suitable metal materials or a combination thereof. Invarious embodiments, the gate electrode layer may be formed by ALD, PVD,CVD, e-beam evaporation, or other suitable process. Accordingly,functional gate structures are formed to wrap around channel members inchannel regions 212C, and include interfacial layers, gate dielectriclayers, and gate electrode layers.

In some embodiments, a planarization process, such as a CMP process, maybe performed to remove excessive materials to provide a substantiallyplanar top surface of the gate structures; gate capping layers, gateself-aligned-contact (SAC) dielectric layers, silicide layers, and/orsource/drain contacts may be formed on the workpiece 200. Referring toblock 126 of FIG. 12 and to FIG. 11, contact features 280 are formed inthe ILD layer 244. In some embodiments, an etching process is employedto remove portions of the ILD layer 244 to form contact trenches. Insome embodiments, the etching operation also removes portions of theCESL 243 such that the source/drain features 245 are exposed in thecontact trenches. In the depicted embodiments, the contact features 280may have a bottom surface that has a width 430 along the X-direction. Insome embodiments, the width 430 is less than the width 410. In suchembodiments, the full dimension of the contact features 280 along theX-direction is utilized such that the contact resistance is minimized.In some embodiments, the width 430 is about 15 nm to about 30 nm. Insome embodiments, a ratio of the width 430 to the width 410 is about1.5:1 to about 3:1. Moreover, in some embodiments, the width 430 isgreater than the width 408. In some approaches, the width 430 is lessthan 408. In such approaches, the size of the contact features 280 maynot have been maximized and the contact resistances between the contactfeatures 280 and the source/drain features 245 may not have beenminimized. In some embodiments, a ratio of the width 430 to the width408 is about 0.5:1 to about 1.2:1.

The methods described above result in several features in the devicesfabricated. For example, the device includes source/drain features 245that have stepped sidewall profiles across the lengthwise direction ofthe gate stacks. A top portion 245B of the source/drain features 245 mayhave a width 410 along the X-direction that is greater than the width408 of the lower portion 245A of the source/drain features 245. The topportion 245B of the source/drain features 245 are partially disposed ontop surfaces of the dielectric features 1216. Moreover, the top portions245B of the source/drain features 245 extend between (and directlycontact) opposing sidewall surfaces of the dielectric features 218.However, the lower portions 245A of the source/drain features 245 extendbetween opposing sidewall surfaces of the dielectric features 1216 andare spaced away from the dielectric features 218. Moreover, the lowerportions 245A and the top portions 245B each interface with dielectricfeatures of different materials. The device also includes contactfeatures 280 having a width 430 along the X-direction. In someembodiments, the width 430 may be greater than the width 408 and lessthan the width 410. In some embodiments, a ratio of the width 410 to thewidth 408 of the channel layers may be about 1.5:1 to about 2:1. Thesource/drain features 245 are connected by the stack of channel layers208. In some embodiments, the topmost channel layer 208 is connected tothe top portion 245B. In some embodiments, the bottommost channel layeris connected to the lower portion 245A of the source/drain features 245.In other words, the source/drain features 245 have a different lateralwidth at the height level of a top surface of the topmost channel layer208 as compared to at the height level of a bottom surface of thebottommost channel layer 208. These device features allow the deviceperformances to be optimized. For example, the source/drain featureshave a larger landing platform for interfacing with the contact featuresfor resistance reductions, and have a smaller bottom dimension forreduced fringe capacitances. By contrast, in approaches not implementingfeatures of the present disclosure, for example, where the source/drainfeatures have substantially straight profiles and uniform lateraldimensions across their respective heights, it may be challenging tosimultaneously optimize the contact resistance as well as the fringecapacitance.

In one exemplary aspect, the present disclosure is directed to a device.The device includes a semiconductor substrate. The semiconductorsubstrate has a substrate surface. The device also includes a stack ofchannel layers on the semiconductor substrate. A top surface of atopmost channel layer of the stack of channel layers extends along afirst height relative to the substrate surface. A bottom surface of abottommost channel layer of the stack of channel layers extends along asecond height relative to the substrate surface. The device furtherincludes a gate structure that engages with the stack of channel layersand extending along a first direction. Additionally, the device includesa source/drain feature on first sidewall surfaces of the stack ofchannel layers and on the substrate, where the first sidewall surfacesextends in parallel to the first direction. Moreover, the source/drainfeature has a first width along the first direction at the first heightand a second width along the first direction at the second height, andwherein the first width is greater than the second width.

In some embodiments, the device also includes a first dielectric featureon second sidewall surfaces of the stack of channel layers. The secondsidewall surfaces extend perpendicular to the first direction. Moreover,the first dielectric feature has a third width along the firstdirection. Furthermore, the third width substantially equals to half ofa difference between the first width and the second width. In someembodiments, the first dielectric feature has a top surface extendingalong a third height above the substrate surface. The third height isgreater than the second height and less than the first height. In someembodiments, the first dielectric feature includes one of siliconcarbonitride (SiCN), silicon carboxynitride (SiCON), and silicon carbide(SiC). In some embodiments, the source/drain feature has a first portionover a second portion and the second portion directly contacts thesubstrate. The first portion has the first width and the second portionhas the second width. Moreover, the first portion has a third heightrelative to the substrate surface, and the second portion has a fourthheight relative to the substrate surface. A ratio of the third height tothe fourth height is about 1.5:0.5 to about 3:2. In some embodiments,the device further includes a second dielectric feature that extendsalong a second direction perpendicular to the first direction. A topportion of the source/drain feature directly contacts the seconddielectric feature, and a bottom portion of the source/drain feature isspaced away from the second dielectric feature. In some embodiments, aratio of the first width to the second width is about 1.5:1 to about2:1. In some embodiments, the stack of channel layers each have a fourthwidth along the first direction, and wherein the fourth width is aboutthe same as the second width. In some embodiments, the source/drainfeature has a sidewall having a stepped profile, the stepped profiledefined by a first feature of a first dielectric material and a secondfeature of a second dielectric materials different from the firstdielectric material.

In one exemplary aspect, the present disclosure is directed to a device.The device includes a semiconductor substrate, a first dielectricfeature, a second dielectric feature, a third dielectric feature, and afourth dielectric feature on the semiconductor substrate. The firstdielectric feature has a first sidewall surface, and the seconddielectric feature has a second sidewall surface, where the firstsidewall surface faces the second sidewall surface. Moreover, the thirddielectric feature is on the first sidewall surface and has a thirdsidewall surface; the fourth dielectric feature is on the secondsidewall surface and has a fourth sidewall surface, where the thirdsidewall surface facing the fourth sidewall surface. The device alsoincludes a source/drain feature over the semiconductor substrate. Thesource/drain feature has a bottom portion that extends between the thirdsidewall surface and the fourth sidewall surface. Furthermore, thesource/drain feature has a top portion that extends between the firstsidewall surface and the second sidewall surface. A first distancebetween the third sidewall surface and the fourth sidewall surface isless than a second distance between the first sidewall surface and thesecond sidewall surface. Additionally, the first dielectric feature andthe second dielectric feature each include a first dielectric material.The third dielectric feature and the fourth dielectric feature eachinclude a second dielectric material. And the second dielectric materialis different from the first dielectric material.

In some embodiments, the top portion of the source/drain featuredirectly contacts a top surface of the third dielectric feature anddirectly contacts a top surface of the fourth dielectric feature. Insome embodiments, a ratio of the first distance to the second distanceis about 1:1.5 to about 1:2. In some embodiments, the third and thefourth dielectric features each have a first height. The top portion ofthe source/drain feature has a second height, and wherein a ratio of thefirst height to the second height is about 1:0.5 to about 1:2.

In one exemplary aspect, the present disclosure is directed to a method.The method includes receiving a semiconductor workpiece. The workpiecehas active regions extending above a top surface of a semiconductorsubstrate. The method also includes forming dielectric features on firstopposing sidewalls of the active regions across a first direction,etching portions of the active region to form source/drain trenches. Thesource/drain trenches exposes second opposing sidewalls of the activeregion. The method further includes recessing the dielectric featuresand forming source/drain features in the source/drain trenches and onthe exposed second opposing sidewalls of the active region. Thesource/drain features is partially formed on top surfaces of thedielectric features.

In some embodiments, the recessing of the dielectric features includesrecessing to reduce a height of the dielectric features relative to thetop surface of the semiconductor substrate. In some embodiments, therecessing of the dielectric features includes recessing to form astepped profile of the source/drain trenches. In some embodiments, theforming of the dielectric feature includes forming a cladding layer onthe first opposing sidewalls of the active regions, where the claddinglayer has exposed sidewall surfaces. The forming of the dielectricfeature also includes forming dielectric barriers on the exposedsidewall surfaces of the cladding layer, removing a portion of thecladding layer between the dielectric barriers and the active regionsthereby forming gaps therebetween, and depositing a dielectric materialinto the gaps. In some embodiments, each of the active region includes astack of first semiconductor layers and second semiconductor layers overthe semiconductor substrate. The first semiconductor layers and thesecond semiconductor layers have different material compositions andalternate with one another within each of the stacks. Moreover, themethod further includes, before the recessing of the dielectricfeatures, replacing a first portion of the second semiconductor layersto form dielectric spacers between end portions of vertically adjacentfirst semiconductor layers. Furthermore, the method includes, after theforming of the source/drain features, removing a remaining portion ofthe second semiconductor layers to form gaps. Additionally, the methodincludes forming metal gate stacks in the gaps. In some embodiments, thefirst semiconductor layers include crystalline silicon, the secondsemiconductor layer includes silicon germanium that has germanium at afirst atomic percentage, and the cladding layer includes silicongermanium that has germanium at a second atomic percentage. The firstatomic percentage is different from the second atomic percentage. Insome embodiments, the recessing of the dielectric features exposes topportions of sidewalls of the dielectric barriers. Moreover, the formingof the source/drain features includes forming on the exposed topportions of the sidewalls of the dielectric barriers.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A device, comprising: a semiconductor substratehaving a substrate surface; a stack of channel layers on thesemiconductor substrate, wherein a top surface of a topmost channellayer of the stack of channel layers extends along a first heightrelative to the substrate surface, and a bottom surface of a bottommostchannel layer of the stack of channel layers extends along a secondheight relative to the substrate surface; a gate structure engaging withthe stack of channel layers and extending along a first direction; asource/drain feature on first sidewall surfaces of the stack of channellayers and on the substrate, the first sidewall surfaces extending inparallel to the first direction; wherein the source/drain feature has afirst width along the first direction at the first height and a secondwidth along the first direction at the second height, and wherein thefirst width is greater than the second width.
 2. The device of claim 1,further comprising a first dielectric feature on second sidewallsurfaces of the stack of channel layers, wherein the second sidewallsurfaces extend perpendicular to the first direction, and wherein thefirst dielectric feature has a third width along the first direction,and the third width substantially equals to half of a difference betweenthe first width and the second width.
 3. The device of claim 2, whereinthe first dielectric feature has a top surface extending along a thirdheight above the substrate surface, and wherein the third height isgreater than the second height and less than the first height.
 4. Thedevice of claim 2, wherein the first dielectric feature includes one ofsilicon carbonitride (SiCN), silicon carboxynitride (SiCON) and siliconcarbide (SiC).
 5. The device of claim 1, wherein the source/drainfeature has a first portion over a second portion, the second portiondirectly contacting the substrate, the first portion having the firstwidth and the second portion having the second width, and wherein thefirst portion has a third height relative to the substrate surface, andthe second portion has a fourth height relative to the substratesurface, and wherein a ratio of the third height to the fourth height isabout 1.5:0.5 to about 3:2.
 6. The device of claim 1, further comprisinga second dielectric feature extending along a second directionperpendicular to the first direction, wherein a top portion of thesource/drain feature directly contacts the second dielectric feature,and a bottom portion of the source/drain feature is spaced away from thesecond dielectric feature.
 7. The device of claim 1, wherein a ratio ofthe first width to the second width is about 1.5:1 to about 2:1.
 8. Thedevice of claim 1, wherein the stack of channel layers each have afourth width along the first direction, and wherein the fourth width isabout the same as the second width.
 9. The device of claim 1, whereinthe source/drain feature has a sidewall having a stepped profile, thestepped profile defined by a first feature of a first dielectricmaterial and a second feature of a second dielectric materials differentfrom the first dielectric material.
 10. A device, comprising: asemiconductor substrate; a first dielectric feature on the semiconductorsubstrate having a first sidewall surface; a second dielectric featureon the semiconductor substrate having a second sidewall surface, thefirst sidewall surface facing the second sidewall surface; a thirddielectric feature on the first sidewall surface and having a thirdsidewall surface; a fourth dielectric feature on the second sidewallsurface having a fourth sidewall surface, the third sidewall surfacefacing the fourth sidewall surface; and a source/drain feature over thesemiconductor substrate having a bottom portion extending between thethird sidewall surface and the fourth sidewall surface and a top portionextending between the first sidewall surface and the second sidewallsurface, wherein a first distance between the third sidewall surface andthe fourth sidewall surface is less than a second distance between thefirst sidewall surface and the second sidewall surface, and wherein thefirst dielectric feature and the second dielectric feature each includea first dielectric material, the third dielectric feature and the fourthdielectric feature each include a second dielectric material, the seconddielectric material being different from the first dielectric material.11. The device of claim 10, wherein the top portion of the source/drainfeature directly contacts a top surface of the third dielectric featureand directly contacts a top surface of the fourth dielectric feature.12. The device of claim 10, wherein a ratio of the first distance to thesecond distance is about 1:1.5 to about 1:2.
 13. The device of claim 10,wherein the third and the fourth dielectric features each have a firstheight, the top portion of the source/drain feature has a second height,and wherein a ratio of the first height to the second height is about1:0.5 to about 1:2.
 14. A method, comprising: receiving a semiconductorworkpiece having active regions extending above a top surface of asemiconductor substrate; forming dielectric features on first opposingsidewalls of the active regions across a first direction; etchingportions of the active region to form source/drain trenches, thesource/drain trenches exposing second opposing sidewalls of the activeregion; recessing the dielectric features; and forming source/drainfeatures in the source/drain trenches and on the exposed second opposingsidewalls of the active region, the source/drain features partiallyformed on top surfaces of the dielectric features.
 15. The method ofclaim 14, wherein the recessing of the dielectric features includesrecessing to reduce a height of the dielectric features relative to thetop surface of the semiconductor substrate.
 16. The method of claim 14,wherein the recessing of the dielectric features includes recessing toform a stepped profile of the source/drain trenches.
 17. The method ofclaim 14, wherein the forming of the dielectric feature includes:forming a cladding layer on the first opposing sidewalls of the activeregions, the cladding layer having exposed sidewall surfaces; formingdielectric barriers on the exposed sidewall surfaces of the claddinglayer; removing a portion of the cladding layer between the dielectricbarriers and the active regions thereby forming gaps therebetween; anddepositing a dielectric material into the gaps.
 18. The method of claim14, wherein each of the active region includes a stack of firstsemiconductor layers and second semiconductor layers over thesemiconductor substrate, wherein the first semiconductor layers and thesecond semiconductor layers have different material compositions andalternate with one another within each of the stacks, the method furthercomprising: before the recessing of the dielectric features, replacing afirst portion of the second semiconductor layers to form dielectricspacers between end portions of vertically adjacent first semiconductorlayers; after the forming of the source/drain features, removing aremaining portion of the second semiconductor layers to form gaps; andforming metal gate stacks in the gaps.
 19. The method of claim 18,wherein the first semiconductor layers include crystalline silicon, thesecond semiconductor layer includes silicon germanium having germaniumat a first atomic percentage, and the cladding layer includes silicongermanium having germanium at a second atomic percentage, and whereinthe first atomic percentage is different from the second atomicpercentage.
 20. The method of claim 18, wherein the recessing of thedielectric features exposes top portions of sidewalls of the dielectricbarriers, and wherein the forming of the source/drain features includesforming on the exposed top portions of the sidewalls of the dielectricbarriers.